The invention relates to a tilt servo apparatus of an optical disc recording/reproducing apparatus and, more particularly, to a liquid-crystal tilt servo apparatus having a liquid crystal device for correcting a tilt error.
Description of the Related Art
In a recording/reproducing apparatus of an optical disc such as CD (Compact Disc), DVD (Digital Versatile Disc), or the like, a deviation occurs between an optical axis of a laser beam irradiated from an optical pickup and a direction normal to the optical disc at the beam irradiating position due to a warp or the like of the optical disc. An angle of such deviation is called a tilt angle, and occurs mainly in the direction of radius of the optical disc (hereinafter, referred to as xe2x80x9cradial directionxe2x80x9d), and which causes a coma-aberration or the like of an optical system. When the tilt angle occurs, signal deterioration such as crosstalk to an adjacent track, jitter, or the like appears which exerts adverse influence on the reproducing quality of the optical disc. Since it is necessary to shorten a wavelength of the laser beam and to increase a numerical aperture NA of an objective lens in order to reduce a spot diameter of the laser beam, particularly, in such case as to perform a high density recording such as in a DVD system, a margin for the tilt angle decreases. Thus, the reproducing quality is greatly deteriorated even if the optical disc is slightly inclined. To correct an aberration due to the tilt angle during the reproduction of the optical disc, therefore, a disc player is generally provided with a tilt servo mechanism for correcting the tilt error on the basis of a detection signal intensity of the reflected laser beam.
As one of such tilt servo systems, there is a liquid-crystal tilt servo apparatus using a liquid crystal device. The principle of the liquid-crystal tilt servo is that a wave front aberration occurring due to a tilt of an optical disc can be compensated by controlling a liquid crystal, which is provided in the optical pickup, to cancel a phase change of the laser beam. A liquid-crystal tilt servo apparatus using a liquid crystal device for compensating the wave front aberration has been disclosed in Japanese Patent Application Kokai No. 10-172163 filed by the present applicant.
Generally, since a so-called surface oscillation occurs owing to a warp of an optical disc, an inclination of a turntable, or the like in an optical disc recording/reproducing apparatus, detection signal intensity fluctuates in accordance with a period of the surface oscillation. In the conventional tilt servo apparatus, however, there are such problems that accurate control cannot be performed and that a long convergence time is required for controlling an optical pickup to the optimum position since the servo control is performed on the basis of the detection signal having a fluctuation due to the surface oscillation.
The invention is made in consideration of the problems and it is an object of the present invention to provide a tilt servo apparatus which performs an accurate and stable tilt correction without being influenced by the surface oscillation of an optical disc.
A tilt servo apparatus according to the present invention comprises: photodetecting means for detecting a reflected light beam of a laser beam irradiated to an optical recording medium and for generating a detection signal, tilt error correcting means for correcting a tilt error occurring due to a tilt angle between an optical axis of the laser beam and a direction normal to the optical recording medium at the laser beam irradiating position, control means for controlling a correction amount of the tilt error correcting means, rotation signal generating means for generating a rotation signal synchronously with a rotation of a motor for rotating the optical recording medium, sampling means for executing a sampling of an envelope signal intensity of the detection signal at every predetermined rotational angle on the basis of the rotation signal; and calculating means for calculating an average value of the sampled envelope signal intensities. The control means determines the correction amount on the basis of the average value.